Many people use mobile stations, such as cell phones and personal digital assistants, to communicate with cellular wireless networks. These mobile stations and networks typically communicate with each other over a radio frequency (RF) air interface according to a wireless protocol. Mobile stations typically conduct wireless communications with one or more remote entities, such as base stations, base transceiver stations, base station controllers, radio network controllers, or access nodes. Each remote entity may be arranged to send communications to and receive communications from mobile stations over the RF air interface. Each remote entity may also provide the mobile station with access to other networks to which the remote entity is connected.
A particular instance of a mobile station communicating with a remote entity may be referred to as a session. In general, a mobile station that has a session with a remote entity can engage in wireless communication with any networks to which the remote entity provides access.
In order to initiate a session, the mobile station may request an air-interface connection from the remote entity, and the remote entity will responsively work to establish the air-interface connection with the mobile station. Once the session is established, the mobile station and the remote entity may exchange communications.
As the mobile station and the remote entity exchange communications, in some instances it may be the case that the remote entity is transmitting communications to be received by the mobile station. In this case, the remote entity may be referred to as a transmitting entity, while the mobile station may be referred to as a receiving entity. The communications may then be referred to as forward-link communications. Alternately, in some instances it may be the case that the mobile station is transmitting communications to be received by the remote entity. In this case, the mobile station may be referred to as the transmitting entity, while the remote entity may be referred to as the receiving entity. The communications may then be referred to as reverse-link communications. For both forward- and reverse-link communications, an amount of power used by a transmitting entity to transmit communications to a receiving entity may be referred to as a power level.
In general, the power level may take a range of values, though values at both ends of the range may have a mixed impact on the session. For example, a higher power level will generally improve the quality of a signal received by the receiving entity. However, the higher power level may also contribute to an increase in overall consumption of power by the transmitting entity, as well as interference with other communications being transmitted over the RF air interface. In contrast, a lower power level may avoid such unnecessary power consumption and interference, but may also result in an inferior signal quality, as compared with the higher power level.
Accordingly, it may be desirable during a session to identify an optimal power level. The optimal power level may, for example, be a power level at which acceptable signal quality may be maintained while minimizing power consumption, interference, and other negative effects of using a higher power level. In order to identify such an optimal power level, the transmitting entity and the receiving entity may engage in what is known as a power control process. A typical power control process works to identify optimal values for both (i) the power level at the transmitting entity and (ii) a setpoint used by the receiving entity to evaluate a determined strength of received communications.
During the session, the transmitting entity may transmit communications to the receiving entity at a given power level. As the receiving entity receives the communications, the receiving entity may periodically determine the strength of the received communications, such as by determining a signal-to-noise ratio (SNR) of the received communications. The receiving entity may then compare the determined strength with the setpoint. This comparison allows the receiving entity to assess the appropriateness of the power level. In particular, the receiving entity may determine whether the power level is too high or too low. Based on the comparison, the receiving entity may instruct the transmitting entity to either increase or decrease the power level in an effort to adjust the power level closer to the optimal power level.
As an example, the comparison may indicate that the received communications are rather weak, such that the receiving entity is not adequately receiving the communications. In this case, the receiving entity may determine that the power level is too low, and may instruct the transmitting entity to increase the power level. Generally, such an increase in the power level will result in an increase in quality for subsequently received communications. This increase in quality may help to ensure that an acceptable signal quality is maintained during the session.
As another example, the comparison may instead indicate that the received communications are unnecessarily strong, such that if the quality of the communications decreased, the receiving entity would still adequately receive the communications. In this case, the receiving entity may determine that the power level is too high, and may instruct the transmitting entity to decrease the power level. Generally, such a decrease in the power level will help to lessen any adverse effects on power consumption and interference resulting from the unnecessarily high power level.
Such increasing and decreasing of the power level may continue repeatedly during the session. With each repetition, the power level will be adjusted closer to the optimal power level.
Also during the session, as the receiving entity receives the communications, the receiving entity may periodically determine an error level of the received communications, such as by determining a frame error rate (FER) of the received communications. Based on the error level of the received communications, the receiving entity may either increase or decrease the setpoint.
As an example, the error level may indicate that the receiving entity is not adequately receiving the communications. In this case, the receiving entity may increase the setpoint. As the setpoint is used by the receiving entity to evaluate the determined strength of received communications, the increased setpoint may, in subsequent repetitions, trigger the receiving entity to instruct the transmitting entity to increase the power level. As discussed above, such an increase in the power level will generally result in an increase in quality for subsequently received communications. This increase in quality may help to ensure that an acceptable signal quality is maintained during the session.
As another example, the error level may instead indicate that the receiving entity is receiving unnecessarily high quality communications, such that if the quality of the communications decreased, the receiving entity would still adequately receive the communications. In this case, the receiving entity may decrease the setpoint. The decreased setpoint may, in subsequent repetitions, trigger the receiving entity to instruct the transmitting entity to decrease the power level. Generally, such a decrease in the power level will help to lessen any adverse effects on power consumption and interference resulting from the unnecessarily high power level.
Such increasing and decreasing of the setpoint may continue repeatedly during the session, triggering subsequent increasing or decreasing of the power level. With each repetition, the setpoint will be adjusted closer to the optimal setpoint, and the power level will be adjusted closer to the optimal power level.
As the transmitting entity transmits communications to be received by the receiving entity, the communications may follow multiple paths between the transmitting entity and the receiving entity. Some paths may be direct paths, meaning the communications follow a substantially straight line between the transmitting entity and the receiving entity. Other paths may be reflected paths, meaning the communications may be reflected off of obstructions one or more times before arriving at the receiving entity.
As a result of the reflections, the communications along the reflected paths may experience phase shifts, time delays, and other undesired alterations. At points in space between the transmitting entity and the receiving entity, the altered communications on the reflected paths and the communications on the direct path may destructively interfere with one another, resulting in what is known as multi-path fading. Multi-path fading may result in a degraded or even unusable reception of the communications at the receiving entity.
One option for combating multi-path fading is receive diversity. In receive diversity, the receiving entity uses two or more antennas to receive the communications transmitted by the transmitting entity. Each antenna may be distinguished from the other antenna(s) by one or more characteristics, such as its polarization, height, position, or radiation pattern (that is, the sensitivity of the antenna as a function of direction). As a result of these distinguishing characteristics, each of the two or more antennas may observe different degrees of multi-path fading. In some cases, one antenna may observe a much lower degree of multi-path fading than the other antennas, allowing the one antenna to receive the communications in a less degraded form.
As an example, the receiving entity may use two antennas. The antennas may have orthogonal polarizations, meaning the first antenna may be oriented in a first plane, while the second antenna may be oriented in a second plane orthogonal to the first plane. Typically, multi-path fading in the first plane will be independent of the multi-path fading in the orthogonal plane. Accordingly, as a result of the orthogonal polarization, the two antennas may observe different degrees of multi-path fading. In some cases, the second antenna may observe a much lower degree of multi-path fading than the first antenna, allowing the second antenna to receive the communications in a less degraded form.
Thus, through the use of two or more antennas, a receiving entity may receive two or more receptions of the same communications, one of which may be stronger than the other(s). In some cases, the receiving entity may simply select the strongest reception for processing. Such a technique may be referred to as selecting. In other cases, the receiving entity may combine the receptions at each of the antennas together for processing. This technique may be referred to as combining. In either case, receive diversity may improve the ability of the receiving entity to receive communications transmitted by the transmitting entity.